1. Field of the Invention
This invention relates to a technique for driving a plasma display, and more particularly to a plasma display driving method and apparatus that is adaptive for driving the plasma display at a high speed as well as improving a contrast.
2. Description of the Related Art
Generally, a plasma display radiates a phosphorus material using an ultraviolet ray generated upon discharge of a gas such as He+Xe or Ne+Xe, to thereby display a picture including characters or graphics. Such a plasma display is easy to be made into a thin-film and large-dimension type. Moreover, the plasma display provides a very improved picture quality owing to a recent technical development.
Particularly, a three-electrode, alternating current (AC) surface-discharge type plasma display has advantages of a low-voltage driving and a long life in that it can lower a voltage required for a discharge using wall charges accumulated on the surface thereof during the discharge and protect the electrodes from a sputtering caused by the discharge.
Referring to FIG. 1, a discharge cell of the three-electrode, AC surface-discharge plasma display includes a scan electrode 30Y and a sustain electrode 30Z formed on an upper substrate 10, and an address electrode 20X formed on a lower substrate 18.
The scan electrode 30Y and the sustain electrode 30Z includes a transparent electrode 12Y or 12Z, and a metal bus electrode 13Y or 13Z having a smaller line width than the transparent electrode 12Y or 12Z and provided at one edge of the transparent electrode, respectively. The transparent electrodes 12Y and 12Z are formed from indium-tin-oxide (ITO) on the upper substrate 10. The metal bus electrodes 13Y and 13Z are formed on the transparent electrodes 12Y and 12Z from a metal such as chrome (Cr) to thereby reduce a voltage drop caused by the transparent electrodes 12Y and 12Z having a high resistance.
On the upper substrate 10 provided with the scan electrode 30Y and the sustain electrode 30Z, an upper dielectric layer 14 and a protective film 16 are disposed. Wall charges generated upon plasma discharge are accumulated onto the upper dielectric layer 14. The protective film 16 protects the upper dielectric layer 14 from a sputtering of the charged particles generated during the plasma discharge and improves the emission efficiency of secondary electrons. This protective film 16 is usually made from MgO.
The address electrode 20X is formed in a direction crossing the scan electrode 30Y and the sustain electrode 30Z. A lower dielectric layer 22 and barrier ribs 24 are formed on the lower substrate 18 provided with the address electrode 20X. A phosphorous material layer 26 is formed on the surfaces of the lower dielectric layer 22 and the barrier ribs 24. The barrier ribs 24 are formed in parallel to the address electrode 20X to divide the discharge cells physically, and prevents an ultraviolet ray and a visible light generated by the discharge from being leaked into adjacent discharge cells.
The phosphorous material layer 26 is excited and radiated by an ultraviolet ray generated upon discharge to produce any one of red, green and blue color visible lights. An inactive mixture gas, such as He+Xe, Ne+Xe or He+Ne+Xe, for a gas discharge is injected into a discharge space defined between the upper/lower substrate 10 and 18 and the barrier ribs 24.
Such a three-electrode AC surface-discharge plasma display drives one frame, which is divided into various sub-fields having a different emission frequency, so as to realize gray levels of a picture. Each sub-field is again divided into a reset period for uniformly causing a discharge, an address period for selecting the discharge cell and a sustain period for realizing the gray levels depending on the discharge frequency. If it is intended to display a picture of 256 gray levels, then a frame period equal to 1/60 second (i.e. 16.67 msec) is divided into 8 sub-fields SF1 to SF8 as shown in FIG. 2. Each of the 8 sub-field SF1 to SF8 is divided into a reset period, an address period and a sustain period. The reset period and the address period of each sub-field are equal every sub-field, whereas the sustain period and the discharge frequency are increased at a ratio of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field. As the sustain period at each sub-field is differentiated as mentioned above, a gray level of a picture can be implemented.
Such a plasma display driving method is largely classified into a selective writing system and a selective erasing system depending on whether or not there is an light-emission of the discharge cell selected by the address discharge.
The selective writing system turns on the discharge cells selected in the address period after turning off the entire field in the reset period. In the sustain period, a discharge of the discharge cells selected by the address discharge is sustained to thereby display a picture.
In the selective writing system, a scanning pulse applied to the scan electrode 30Y must be set to have a relatively large pulse width, thereby forming sufficient wall charges within the discharge cell.
If the plasma display has a resolution of VGA (video graphics array) class, it has total 480 scanning lines. Accordingly, in the selective writing system, an address period within one frame requires total 11.52 ms when one frame period (i.e., 16.67 ms) includes 8 sub-fields. On the other hand, a sustain period is assigned to 3.05 ms in consideration of a vertical synchronizing signal Vsync. Herein, assuming that a pulse width of the scanning pulse should be 3 μs, the address period is calculated by 3 μs (a pulse width of the scanning pulse)×480 lines×8 (the number of sub-fields) per frame. The sustain period is a time value (i.e., 16.67 ms−11.52 ms−0.3 ms−1 ms−0.8 ms) obtained by subtracting an address period of 11.52 ms, once reset period of 0.3 ms, an erase period of 100 μs×8 sub-fields and an extra time of the vertical synchronizing signal Vsync of 1 ms from one frame period of 16.67 ms.
The plasma display may generate a pseudo contour noise from a moving picture because of its characteristic realizing the gray levels of the picture by a combination of sub-fields. If the pseudo contour noise is generated, then a pseudo contour emerges on the screen to thereby deteriorate a picture display quality. For instance, if the screen is moved to the left after the left half of the screen was displayed by a gray level value of 128 and the right half of the screen was displayed by a gray level value of 127, then a peak white, that is, a white stripe emerges at a boundary portion between the gray level values 128 and 127. To the contrary, if the screen is moved to the right after the left half thereof was displayed by a gray level value of 128 and the right half thereof was displayed by a gray level value of 127, then a black level, that is, a black stripe emerges on at a boundary portion between the gray level values 127 and 128.
In order to eliminate a pseudo contour noise of a moving picture, there has been suggested a scheme of dividing one sub-field to add one or two sub-fields, a scheme of re-arranging the sequence of sub-fields, a scheme of adding the sub-fields and re-arranging the sequence of sub-fields, and an error diffusion method, etc. However, in the selective writing system, if the sub-fields are added so as to eliminate a pseudo contour noise of a moving picture, then the sustain period becomes insufficient or fails to be assigned. For instance, in the selective writing system, if two sub-fields of the 8 sub-fields are divided such that one frame includes 10 sub-fields, then the display period, that is, the sustain period becomes absolutely insufficient as follows. If one frame includes 10 sub-fields, then the address period becomes 14.4 ms, which is calculated by 3 μs (a pulse width of the scanning pulse)×480 lines×10 (the number of sub-fields) per frame. On the other hand, the sustain period becomes −0.03 ms (i.e., 16.67 ms−14.4 ms−0.3 ms−1 ms−1 ms), which is a time value obtained by subtracting an address period of 14.4 ms, once reset period of 0.3 ms, an erase period of 100 μs×10 sub-fields and an extra time of the vertical synchronizing signal Vsync of 1 ms from one frame period of 16.67 ms.
In such a selective writing system, a sustain period of about 3 ms can be assured when one frame consists of 8 sub-fields, whereas it becomes impossible to assure a time for the sustain period when one frame consists of 10 sub-fields. In order to overcome this problem, there has been suggested a scheme of making a divisional driving of one field. However, such a scheme raises another problem of a rise of manufacturing cost because it requires an addition of driver IC's.
A contrast characteristic of the selective writing system is as follows. In the selective writing system, when one frame consists of 8 sub-fields, a light of about 300 cd/m2 corresponding to a brightness of the peak white is produced if a field continues to be turned on in the entire sustain period of 3.05 ms. On the other hand, if the field is sustained in a state of being turned on only in once reset period and being turned off in the remaining interval within one frame, then a light of about 0.7 cd/m2 corresponding to the black is produced. Accordingly, a darkroom contrast ratio in the selective writing system has a level of 430:1.
The selective erasing system makes a writing discharge of the entire field in the reset period and thereafter turns off the discharge cells selected in the address period. Then, in the sustain period, only the discharge cells having not selected by the address discharge are subject to a sustain discharge to thereby display a picture.
In the selective erasing system, a selective erasing data pulse is applied to the address electrode 20X so that it can erase wall charges and space charges of the discharge cells selected during the address discharge. At the same time, a scanning pulse synchronized with the selective erasing data pulse is applied to the scan electrode 30Y. A pulse width of the data pulse and the scanning pulse for causing an erasing address discharge in the selective erasing system may be narrower than that of the data pulse and the scanning pulse in the selective writing system.
In the selective writing system, if the plasma display has a resolution of VGA (video graphics array) class, then an address period within one frame requires only total 3.84 ms when one frame period (i.e., 16.67 ms) consists of 8 sub-fields.
On the other hand, a sustain period can be sufficiently assigned to about 10.73 ms in consideration of a vertical synchronizing signal Vsync. Herein, assuming that a pulse width of the scanning pulse should be 1 μs, the address period is calculated by 1 μs (a pulse width of the scanning pulse)×480 lines×8 (the number of sub-fields) per frame. The sustain period is a time value (i.e., 16.67 ms−3.84 ms−0.3 ms−1 ms−0.8 ms) obtained by subtracting an address period of 3.84 ms, once reset period of 0.3 ms, and an extra time of the vertical synchronizing signal Vsync of 1 ms and an entire writing time of 100 μs×8 sub-fields from one frame period of 16.67 ms. In such a selective erasing system, since the address period is small, the sustain period as a display period can be assured even though the number of sub-fields is increased. If the number of sub-fields SF1 to SF10 within one frame is increased into ten as shown in FIG. 3, then the address period becomes 4.8 ms, which is calculated by 1 μs (a pulse width of the scanning pulse)×480 lines×10 (the number of sub-fields) per frame. On the other hand, the sustain period becomes 9.57 ms, which is a time value (i.e., 16.67 ms−4.8 ms−0.3 ms−1 ms−1 ms) obtained by subtracting an address period of 4.8 ms, once reset period of 0.3 ms, an extra time of the vertical synchronizing signal Vsync of 1 ms and the entire writing time of 100 μs×10 sub-fields from one frame period of 16.67 ms. Accordingly, the selective erasing system can assure a sustain period three times longer than the above-mentioned selective writing system having 8 sub-fields even though the number of sub-fields is enlarged into ten, so that it can realize a bright picture with 256 gray levels.
However, the selective erasing system has a disadvantage of low contrast because the entire field is turned on in the entire writing interval that is a non-display interval.
In the selective erasing system, if the entire field continues to be turned on in the sustain period of 9.57 ms within one frame consisting of 10 sub-fields SF1 to SF10 as shown in FIG. 3, then a light of about 950 cd/m2 corresponding to a brightness of the peak white is produced. A brightness corresponding to the black is 15.7 cd/m2, which is a brightness value of 0.7 cd/m2 generated in once reset period plus 1.5 cd/m2×10 sub-fields generated in the entire writing interval within one frame. Accordingly, since a darkroom contrast ratio in the selective erasing system is equal to a level of 950:15.7=60:1 when one frame consists of 10 sub-fields SF1 to SF10, the selective erasing system has a low contrast. As a result, a driving method using the selective erasing system provides a bright field owing to an assurance of sufficient sustain period, but fails to provide a clear field and a feeling of blurred picture due to a poor contrast.
In order to overcome a problem caused by such a poor contrast, there has been suggested a scheme of making an entire writing only once per frame and taking out the unnecessary discharge cells every sub-field SF1 to SF10. However, this scheme has a problem of poor picture quality in that, since the discharge cell can be selected at the next sub-field only when the previous sub-field has been necessarily turned on, the number of gray levels becomes merely the number of sub-fields plus one. In other words, if one frame includes 10 sub-fields, then the number of gray level becomes merely eleven as indicated by the following table:
TABLE 1GraySF5SF6SF7SF8SF9SF10LevelSF1 (1)SF2 (2)SF3 (4)SF4 (8)(16)(32)(48)(48)(48)(48)0XXXXXXXXXX1◯XXXXXXXXX3◯◯XXXXXXXX7◯◯◯XXXXXXX15◯◯◯◯XXXXXX31◯◯◯◯◯XXXXX63◯◯◯◯◯◯XXXX111◯◯◯◯◯◯◯XXX159◯◯◯◯◯◯◯◯XX207◯◯◯◯◯◯◯◯◯X255◯◯◯◯◯◯◯◯◯◯
In Table 1, ‘O’ represents a state in which a discharge cell is turned on at the corresponding sub-field; and ‘x’ represents a state in which a discharge cell is turned off at the corresponding sub-field.
In this case, since only 1331 colors are expressed by all combination of red, green and blue colors, color expression ability becomes considerably low in comparison to 16,700,000 true colors. The plasma display adopting such a system has a darkroom contrast ratio of 430:1 by a peak white of 950 cd/m2 when the entire field is turned on in the display interval of 9.57 ms and a black of 2.2 cd/m2 which is a brightness value obtained by adding a brightness of 0.7 cd/m2 generated in once reset period to a brightness of 1.5 cd/m2 generate in once entire writing interval.
As described above, in the conventional plasma display driving method, the selective writing system fails to drive the plasma display at a high speed because the data pulse and the scanning pulse for selectively turning on the discharge cells during the address period are relatively wide. The selective erasing system has an advantage in that it can drive the plasma display at a high speed because the data pulse and the scanning pulse for selectively turning off the discharge cells may be narrower than those in the selective writing system, whereas it has a disadvantage of a worse contrast than the selective writing system because the discharge cells at the entire field is turned on in the reset period, that is, the non-display interval.